Universal telephony interface polarity detector

ABSTRACT

A method and apparatus for measuring signals conveyed by a varying the voltage polarity across the tip and ring terminals in a telephone system is disclosed. The invention provides a composite circuit including a first stage in the form of a differential amplifier, a second stage in the form of a low pass filter, and a third stage in the form of a Schmidt trigger. The invention is capable of discerning an intentional polarity reversal telephony signal by eliminating transmission noise, as well as false triggering due to a high frequency (relative to a defined threshold) ring signal.

RELATED APPLICATION

This is a conversion of provisional patent application No. 60/315,797filed Aug. 29, 2001.

TECHNICAL FIELD

This invention relates to telephony, and more particularly, to animproved technique for detecting the polarity of a voltage presentacross a set of telephone wires in both the on-hook and off-hook states.

BACKGROUND OF THE INVENTION

In telephony, line polarity reversal is used as a method of call statussignaling in analog telephony interfaces. This event is used primarilyto frame the beginning and ending of a telephone communicationtransaction, in order to prevent call collisions, known as a glarecondition, in automated telephony equipment. Essentially, line polarityreversal is used to signal the receiving telephone that a call is comingin. In other countries around the world polarity reversals are used tosignal a variety of telephony events.

Another application of polarity reversal involves what is known ascaller identification or caller ID. In a caller ID system, the telephonenumber of the calling party is transmitted to the called telephone anddisplayed on a screen prior to the called telephone being answered. Apolarity reversal can thus provide an indication that caller IDinformation is forth coming.

Conventional implementations of on-hook polarity detection use currentbleed resistors or opto-couplers to sense the polarity. The bleedresistor solution decreases the input impedance (generally undesirable)and is also subject to noise problems. The opto-coupler solutioninvolves an E-O-E conversion; i.e., comprises a first converter thattransforms an electrical current into a light signal, and a secondconverter that changes the light signal back into an electrical current.Such an electric-optic-electric conversion process draws significantcurrent to turn on. While this is tolerable in certain countries whereimpedance limitation requirements have been relaxed specifically forthis type of circuitry, these circuits cannot meet the requirements ofmore stringent countries. Accordingly, using the existing state of theart methods, one cannot build a universal telephony interface polaritydetector, which is capable of meeting the requirements of all countries,whether the current bleed resistor or the opto-coupler system isemployed.

In view of the widespread use of polarity modulation (i.e., the varyingof the polarity of a voltage across the tip and ring inputs to atelephone line) to transmit data to telephone receiving equipment, aneed exists in the art for an improved technique of accurately detectingthe polarity of a voltage, and more importantly, a shift in saidpolarity, across telephone equipment. Preferably, such an improvedtechnique would allow an input impedance that is high enough to satisfythe impedance limitation requirements of all countries, including thosewith the most stringent standards. Such a technique would alsopreferably be impervious to noise problems, and operable down to arelatively low voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a portion of a circuit diagram for use in practicing thepresent invention.

FIG. 2A is a plot of the output voltage verses the differential inputvoltage for the first stage of the circuit presented in FIG. 1.

FIG. 2B a depiction of the output voltage verses input voltage of thethird stage of the circuit depicted in FIG. 1.

FIG. 2C is a plot of the overall polarity output verses the differentialinput of the circuit depicted in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a portion of a circuit diagram for use in practicing thepresent invention. The arrangement of FIG. 1 includes only the portionof circuitry that detects a differential voltage between inputs 101 and102 corresponding to the tip and ring inputs to a standard telephonicdevice.

As can be readily determined from FIG. 1, the circuit diagram presentedconsists of three stages. The first stage, extending from inputs 101 and102 to node A, comprises a differential amplifier. Inputs 101 and 102represent the tip and ring signals entering a standard telephonicdevice. Each of these inputs 101 and 102 is connected respectively to ahigh impedance input resistor 104 and 105. From there, the input signalsare connected respectively to the inverting and non-inverting terminalsof an operational amplifier (op amp) 110. The op amp 110 has apredetermined open loop gain, but, as depicted in FIG. 1, there is afeedback loop to the inverting input terminal through gain resistor 103.This configuration allows the closed loop gain to be directlyproportional to the ratio of gain resistors 103 to 104 and 105.Resistors 103, 104 and 105 can be set as desired by the particularapplication of a given user, however according to the preferredembodiment the closed loop gain is small and the impedance of each ofthe sense inputs is sufficiently high so as to meet the impedancelimitation requirements of the most stringent countries' telephonesystems. A pair of diodes 111 and 112 is connected to the junctionbetween resistors 103 and 104 to provide overvoltage protection to theop amp 110.

As well, diodes 111 and 112 are connected in such a manner so as tolimit the op amp input voltage to no more than 0.3 volts above thepositive power rail voltage and no more than 0.3 volts below thenegative power rail voltage applied to op amp 110. The switching speedsof diodes 111 and 112 are preferably fast.

Sense input 102 is fed into resistor 105, the output from which connectsto the non-inverting terminal of op amp 110. An additional connectionfrom the non-inverting terminal of operational amplifier 110 leadsthrough resistor 115 to ground, connecting to a pair of diodes 113 and114 in mid-path. The combination of diodes 113 and 114 providesovervoltage protection to op amp 110. All diodes in the system 111–114are preferably fast switching diodes to provide transient voltageprotection.

The first stage of the invention circuit is completed at node A inFIG. 1. At node A the output voltage will be equal to the ratio ofclosed loop gain resistors 103 and 104 multiplied by the differentialvoltage between the tip and ring inputs 101 and 102, respectively. Inthe illustrated example, resistors 103, 104, 105, and 115 each have avalue of 10 MΩ. For purposes of simplicity, in the following discussionit will be assumed that the closed loop gain is in fact unity and thatstage 1 functions essentially as an amplifier with a gain of 1 and ahigh input impedance, and that is output voltage limiting. The resultingresponse of output voltage as a function of input voltage for the firststage in the circuit of FIG. 1 is plotted in FIG. 2A, and shows atransfer function of unity gain. Changes in the values of variouscomponents in the circuit of FIG. 1 will obviously result in a differentresponse curve.

The second stage of the circuit depicted in FIG. 1 begins at node A andterminates at node B. The second stage circuit segment is essentially anRC network functioning as an analog low pass filter. The low pass filteroperates here as a false detection filter. The RC network comprisesresistor 106 and grounded capacitor 130, connected to one another atnode B. The values of resistor 106 and capacitor 130 are chosen suchthat all signals detected at or above a minimum frequency will befiltered out. The reason for this second stage is simple. During a ringsignal, the polarity across the tip and ring terminals necessarilyoscillates. Such polarity reversals are not the ones used for signalingthat the present invention is designed to detect, and as such they mustbe discarded. The values of resistor 106 and capacitor 130 can thus bechosen by the user as desired to comply with this functionality. In theexample disclosed herein, resistor 106 is set at 1.87 MΩ and capacitor130 is 0.022 μF, thus forming an analog low pass filter with a timeconstant of RC=0.04114 seconds. The resulting low pass filter thus has a3-dB frequency of approximately 3.86 Hz. As an example, one can choosethe resistor 106 and capacitor 130 such that the pass band of the analogfrequency filter is in the range from 0 to 7 Hz. In a telephonic systemwhere the lowest ring frequency is 15 Hz this choice of pass band issatisfactory.

The third stage of the circuit depicted in FIG. 1 begins at node B andends at the polarity output 150. The third stage is essentially aSchmidt trigger, or an analog bi-stable multivibrator. As will bedescribed in more detail below, the bi-stable multivibrator has twostable states representing the high and low voltages, limited only bythe saturation of op amp 120. Accordingly, the output of the third stageof the circuit of FIG. 1 will be approximately 5 volts (or ground) whereop amp 120 is powered by +/−5 volt power rails.

The third stage of the circuit of the invention operates as follows. Theeffect of a voltage at the non-inverting terminal of op amp 120 dependson the previous state of its output, depicted as polarity output 150 inFIG. 1. If a shift in polarity occurs and continues long enough tocharge capacitor 130 in stage 2 of the circuit, and if the voltage is ofan amplitude large enough to shift stage 3 into its other stable state,then the polarity output 150 will also be reversed. If the polarityremains the same, then polarity output 150 will simply remain constant.The output at 150 is monitored by a microprocessor (not shown) that isprogrammed to register signal polarity at 10 ms intervals.

In the third stage of the circuit, beginning at node B, resistor 107 isconnected in series to the inverting terminal of operational amplifier120. The non-inverting terminal of op amp 120 connects through resistor109 to ground. Resistor 108 forms a positive feedback loop between opamp 120 output and the non-inverting input thereof. The output of op amp120 also connects through diode 127 to the polarity output 150 andthrough resistor 125 to ground. According to the preferred embodiment,resistor 107 is 825Ω, resistor 108 is 1 KΩ, resistor 109 is 2 KΩ andresistor 125 is 10 KΩ. Diode 127 preferably fires at less than one volt.Op amp 120 is powered with voltage rails of +/−5V.

In view of the above-described operation of the circuit of the presentinvention, there is a polarity output 150, which takes one of twopossible states. There is a high state and a low state, eachcorresponding to the maximum high of op amp 120 in saturation or a low,determined by pull-down resistor 125. Thus there is essentially adigital polarity indicator taking either a high or a low state, whichrejects false triggering due to ring voltage-induced polarityinversions, and has a sufficiently broad dead band to prevent unwantedtransitions in the event that battery voltage drops, the line isdisconnected, or the loop current drops. Because the circuit presents avery high input impedance, the circuit can meet the requirements ofvirtually all international PSTN country requirements.

The circuit is set so as to reject polarity reversals of less than 33.33ms duration—corresponding to a half cycle of a minimum ring frequency of15 Hz. Design considerations and alternative ring frequency conventionswill dictate variation from this value as may be necessary in otherembodiments. As well, the circuit is set so as to reject polarityreversals, even if of sufficient duration, if the filtered voltage doesnot reach at least ⅔ of the opposite Vsat. In the circuit, where theVsat range is +/−5V, the voltage at point B in FIG. 1 must reach atleast 3.33V in the opposite direction in order for a polarity reversalto be registered as same and change the polarity output 150 of thecircuit.

FIG. 2A graphically illustrates the output voltage from the first stageof the described circuit as a function of differential input voltage. Asshown, the voltage curve goes from a −5V input differential voltage and−5V output voltage to a +5V input differential voltage and +5V outputvoltage along a substantially straight slope. FIG. 2B shows the thirdstage output voltage 150 in response to the hysteresis effect of theSchmidt trigger. The Schmidt trigger causes the output voltage to remainat its supply value until reaching a triggering differential at V_(TH)voltage threshold (+ or −) and then change to its new output valueprecipitously. FIG. 2C provides a plot of input sense voltage acrossterminals 101 and 102 (see FIG. 1) and a plot of output voltage atpolarity output terminal 150. Plot C is the Vin signal, and Plot D theVout signal. These two voltages are plotted versus time; thus the x-axisrepresents time in FIG. 2C, while the y-axis represents voltage, and theplots C and D are superimposed in the figure.

The input voltage C changes polarity definitively at locations G and Hon FIG. 2C, with the undulating voltage of a ring signal R notconsidered by the system. In parallel with, and slightly delayed fromthe illustrated polarity changes, the output voltage 150 is shown as(plot D in FIG. 2C) increasing from approximately zero volts to apositive value and falling back to zero thereafter.

As described, according to the preferred embodiment circuit describedabove, the false detection filter of stage 2 and the Schmidt trigger ofstage 3 operate together to eliminate voltage polarity transitions ofless than 67 ms duration caused by the maximum ring voltage at a minimumring frequency of 15 Hz and a minimum DC battery voltage. The full pulsecycle a of ring signal r coordinates with the filter limit of 67 ms:actually the value is 66⅔ (or 66.67 in decimal notation) for a safetyfactor of double, since one-half the 15 Hz cycle will have a duration of33.33 ms. Other time thresholds (due to varying ring conventions, or alesser desired safety margin) that are longer or shorter than 67 ms maybe selected by varied system design, especially in the choice ofcomponent values in the Stage 2 resistor 106 and capacitor 130 withreference to FIG. 1.

Thus, as depicted in FIG. 2C, the circuit of the preferred embodimentoperates to eliminate both (i) voltage spikes S with a maximum valuegreater than the minimum voltage and a duration of less than 67 ms, aswell as (ii) polarity crossovers T having lower than the minimum voltageyet extending longer than the 67 ms time bar. As seen in FIG. 2C,neither voltage spikes S of short duration and high voltage nor polaritycrossovers such as T, having long duration and low voltage, areinterpreted by the circuit as a valid polarity reversal. The circuitonly permits a polarity reversal that exceeds both the desired voltageand temporal minima to be registered.

Besides the ability to detect the polarity of the tip/ring pair of aloop start interface in the on-hook condition, the circuit of thepresent invention is capable of detecting the direction of loop currentflow in an off-hook condition. The circuit thus provides a reliable callcollision avoidance mechanism and also provides off-hook polarity sense.

While the foregoing describes the preferred embodiment of the invention,it will be appreciated by those of skill in the art that variousmodifications and additions may be made. Such modifications wouldinclude replacing the circuit components at each stage with functionallyequivalent apparatus, as well as combining various stages or the entirecircuit on a single integrated circuit. Such additions and modificationsare intended to be covered by the following claims.

1. A circuit for detecting a reversal in polarity, comprising: adifferential amplifier; a low pass filter connected in series with theamplifier; and a Schmidt trigger connected in series with the low passfilter.
 2. The circuit of claim 1, wherein the differential amplifiercomprises an operational amplifier having a feedback loop from an outputterminal thereof to an inverting input terminal thereof.
 3. The circuitof claim 1, wherein the Schmidt trigger comprises an operationalamplifier having a feedback loop from an output terminal thereof to anon-inverting input terminal thereof.
 4. A method for detecting apolarity reversal in a telephony circuit comprising: providing adifferential input voltage across the inputs of a differentialamplifier; providing a low pass filter connected to an output of thedifferential amplifier; providing a Schmidt trigger connected to anoutput of the low pass filter; and determining polarity stasis orreversal based upon an output of the Schmidt trigger.
 5. The method ofclaim 4, wherein the step of providing a differential input voltageacross a differential amplifier comprises providing said voltage acrossan operational amplifier having a feedback loop from an output terminalthereof to an inverting input terminal thereof.
 6. The method of claim4, wherein the step of providing a Schmidt trigger comprises providingan operational amplifier having a feedback loop from an output terminalthereof to a non-inverting input terminal thereof.
 7. The method ofclaim 4, further comprising eliminating voltage polarity transitionsthat are shorter than a defined time.
 8. A method for detecting apolarity reversal in a telephony circuit comprising: comparing therelative voltage of two inputs; filtering out a polarity reversal thatlasts shorter than a defined time; and filtering out polarity reversalswhere the final relative voltage is below a defined threshold.
 9. Themethod of claim 8, where the defined time is such so as to filter outany polarity reversal induced by an incoming ring signal.
 10. The methodof claim 8, where the defined threshold is such so as to filter out anypolarity reversal caused by any of battery voltage drops, linedisconnections, or loop current drops.
 11. An apparatus for detecting areversal in polarity, comprising: a comparator; a low pass filterconnected to an output of the comparator; and a hysteresis elementconnected to an output of the low pass filter.
 12. The apparatus ofclaim 11, wherein the hysteresis element comprises a Schmidt trigger.13. The apparatus of claim 11, where the low pass filter is designed tofilter out any polarity reversal induced by an incoming ring signal. 14.The apparatus of claim 11 where the hysteresis element is such so as tofilter out any polarity reversal caused by any of battery voltage drops,line disconnections, or loop current drops.
 15. The apparatus of claim13 where the hysteresis element is such so as to filter out any polarityreversal caused by any of battery voltage drops, line disconnections, orloop current drops.